For the existing HIV-preventing medications we have, such as PrEP, the treatments are much more effective in men than women. It can easily take a month for PrEP to be effective in Assigned Female At Birth folks, but I saw studies during the COVID era that a few days were plenty for Assigned Male At Birth folks. There's some logic to focusing on the cohort that usually has the lowest success rate with these kinds of medication.

The difference in efficacy has to do with the way the body is designed to absorb fluids, and its one of the few areas where hormones and surgery don't make a difference. (A trans guy on HRT with a full set of surgeries generally has male chances of cardiovascular issues, male levels of calorie consumption/muscle-building, etc.)